Thin film strain gauge

ABSTRACT

A strain gauge includes: a substrate; a dielectric layer on the substrate; a thin film electrical circuit on the dielectric layer and having input/output terminals; other layers disposed on the electrical circuit; the dielectric layer forming a first seal on one side of the electrical circuit, the other layers forming a second seal on a second side of the electrical circuit, the first and second seals having structure such that: in a first instance prior to exposure of the strain gauge to an autoclave cycle, the electrical circuit is productive of a first output voltage in response to a first input voltage; and in a second instance subsequent to exposure of the strain gauge to at least 25 autoclave cycles, the electrical circuit is productive of a second output voltage in response to a second input voltage, the first and second input voltages being equal, and the first and second output voltages being equal.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to a strain gauge, particularlyto a thin film strain gauge, and more particularly to an autoclavablethin film strain gauge.

Thin film strain gauges employing precision resistors in a Wheatstonebridge electrical circuit arrangement are known in the art.

Applicant has been using thin film vacuum deposition process to bondstrain gauges directly to 15-5 stainless steel, Inconel, sapphire, andtitanium for a number of years. A typical process begins by preparingthe surface of the substrate with an abrasive slurry to remove allsurface imperfections. The next step is the deposition of an oxide layerto insulate the circuit from the metal substrate. Following this, a thinfilm resistive alloy is sputtered over the oxide layer. This latter filmis laser trimmed under power to produce the four resistors of theWheatstone bridge. Solder pads are applied and wired to the circuit toprovide a power egress and the whole thing is coated with anencapsulation to protect the thin film.

Over time, the thin film strain gauge has proven itself to be thepreferred means for measuring strain in critical applications wheresmall size, robust performance, long term stability and superioraccuracy are required. An application of the thin film sensor technologyis in the field of medical pump technology. When delivery of fluids tothe body via infusion pumps, insulin pumps, enteral feed pumps, andwound irrigation systems is interrupted by a pinched tube or pump,undesired consequences may result. Often, ‘tube sensors’ are used tomonitor pressure in these pump systems by measuring the force exertedonto a sensor pressed against the expanding walls of a polyurethane orPVC tubing or they place the sensor behind the pump to record pressuresas the pump backs up against the sensor during operation. Thin filmsensors have the repeatability and the ability to survive the roughhandling and accuracy required to be successful in these pumpapplications.

However, such pump applications do not involve the surgicalsterilization of a surgical device having a thin film strain gauge thatis an integral part of the surgical device.

As such, and while existing thin film sensors may be suitable for theirintended purpose, the art of thin film strain gauges would be advancedwith a thin film strain gauge having the structural integrity towithstand exposure to multiple autoclave surgical-instrument sterilizingcycles and multiple surgical-instrument cleaning detergent cycleswithout statistically significant loss in accuracy.

This background information is provided to reveal information believedby the applicant to be of possible relevance to the present invention.No admission is necessarily intended, nor should be construed, that anyof the preceding information constitutes prior art against the presentinvention.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment includes an open-face strain gauge, having: a substrate;at least one dielectric layer disposed on top of the substrate; a thinfilm electrical circuit disposed on top of the at least one dielectriclayer and having at least two input terminals and at least two outputterminals; a plurality of layers disposed on top of the electricalcircuit; the at least one dielectric layer forming a first moistureresistant seal on one side of the electrical circuit, the plurality oflayers forming a second moisture resistant seal on a second side of theelectrical circuit opposite the first side, the first and secondmoisture resistant seals having structure such that: in a first instanceprior to exposure of the strain gauge to an autoclave cycle, theelectrical circuit is productive of a first output voltage on the outputterminals in response to a first input voltage on the input terminals;and in a second instance subsequent to exposure of the strain gauge toat least 25 autoclave cycles, the electrical circuit is productive of asecond output voltage on the output terminals in response to a secondinput voltage on the input terminals, the second input voltage beingequal to the first input voltage, and the second output voltage beingequal to the first output voltage.

The above features and advantages and other features and advantages ofthe invention are readily apparent from the following detaileddescription of the invention when taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary non-limiting drawings wherein like elementsare numbered alike in the accompanying Figures:

FIG. 1 depicts a generalized cutaway rotated isometric view of anexample embodiment of a thin film strain gauge in accordance with anembodiment of the invention;

FIG. 2 depicts a top-down plan view of the strain gauge of FIG. 1 inaccordance with an embodiment of the invention;

FIG. 2A depicts an example Wheatstone bridge in accordance with anembodiment of the invention;

FIG. 3 depicts a section cut through Section 3-3 in FIG. 2 in accordancewith an embodiment of the invention; and

FIG. 4 depicts an alternative section cut similar to that of FIG. 3 inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specifics forthe purposes of illustration, anyone of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Accordingly, the followingexample embodiments of the invention are set forth without any loss ofgenerality to, and without imposing limitations upon, the claimedinvention.

An embodiment of the invention, as shown and described by the variousfigures and accompanying text, provides an open-face thin film straingauge having the structural integrity to withstand exposure to multipleautoclave surgical-instrument sterilizing cycles and multiplesurgical-instrument cleaning detergent cycles without statisticallysignificant loss in accuracy. In an embodiment, the structural integrityis provided by an arrangement of layers (discussed further below) thatprovide a bonded moisture seal on both bottom and top surfaces of astrain sensitive layer, while having the flexibility to move with thestrain sensitive layer, without disturbing the bonded moisture seals,during a strain measurement action.

As used herein, the term open-face thin film strain gauge in generalmeans a strain gauge having a strain sensitive layer that is notprotected from the surrounding environment by way of an hermeticallysealed cap, which may be metal, ceramic, plastic, or any other suitablematerial, for example. Other features associated with the term open-facethin film strain gauge will become apparent from the description thatfollows.

While example embodiments disclosed herein depict a strain gauge havinga circular footprint, it will be appreciated that the disclosedinvention is not so limited and encompasses other shapes of straingauges, such as rectangular, octagonal, or otherwise non-circular, forexample. Any and all shapes of strain gauges that fall within the ambitof the appended claims are considered to be within the scope of theinvention disclosed herein.

FIG. 1 depicts generally (some detail omitted for clarity but elaboratedon in other figures) a cutaway rotated isometric view of an exampleembodiment of a thin film strain gauge 100 (also herein referred tosimply as strain gauge) having a substrate 102 and a plurality of layers104 disposed on top of the substrate 102. While the plurality of layers104 are depicted in bulk in FIG. 1, they are depicted and described inmore detail below with reference to other figures. As depicted in FIG.1, an embodiment of the substrate 102 has an overall circular footprinthaving a central Z-axis, and an engagement surface 106 disposed at anunderside region of the substrate 102.

In an embodiment, the substrate 102 comprises a flexible corrosionresistant metal such as titanium, ceramic, aluminum, stainless steel,precipitation-hardening stainless steel, or a superalloy, for example.

As used herein, any reference to top, bottom, upper, lower, underside,upperside, inside, outside, or any other orientational language, is notintended to be limiting in any way, but is merely used herein to orientcertain features relative to other features, or axes, such as theorthogonal set of X-Y-Z axes depicted in FIG. 1.

The upperside 108 of the strain gauge 100 is in open-face relationshipwith ambient, that is, the upperside 108 is not protected from ambientby way of an hermetically sealed cap, which is a known method in the artfor protecting the strain sensitive layer of a strain gauge. Such aknown method, however, results in a physically larger strain gauge thatmay be undesirable or unusable in certain strain gauge applications.

On the upperside 108, and part of the composite of the plurality oflayers 104 is an embedded strain sensitive layer in the form of a thinfilm electrical circuit comprising an electrically conductive strainsensor layer 300 (best seen with reference to FIGS. 2 and 3 anddiscussed in more detail below) having a plurality of electricallyresistive thin film sensors 320 that form a Wheatstone bridge 322 (bestseen with reference to FIGS. 2 and 2A and discussed in more detailbelow). The sensors 320 of the Wheatstone bridge 322 are disposedgenerally within the region 110 bounded by the dashed lines depicted inFIG. 1.

While an embodiment of the strain gauge 100 is disclosed herein havingthe strain sensor layer 300 disposed on the upperside 108 of thesubstrate 102, it will be appreciated that this is for illustrationpurposes only and is non-limiting, as the strain sensor layer 300 mayalternatively be disposed on the underside 107 of the substrate 102.Stated alternatively, the strain sensor layer 300 may be disposed on aside of the substrate 102 that is directly subjected to a force F orpressure P, or may be disposed on an opposing side of the substrate 102that is not directly subjected to the force F or pressure P.

In an embodiment, the electrically conductive strain sensor layer 300comprises a resistive alloy, and the electrically resistive thin filmsensors 320 of the Wheatstone bridge 322 comprise electrical pathsdefined by etched or laser trimmed portions of the resistive alloy (bestseen with reference to FIG. 2).

FIG. 2 depicts a top-down plan view of an example embodiment of thestrain gauge 100 (with the electrically conductive strain sensor layer300 depicted as though it were visible) having eight electricallyresistive thin film sensors 320.1-320.8 (generally referred to incombination by reference numeral 320) electrically arranged to form theWheatstone bridge 322 (see FIG. 2A), and FIG. 3 depicts a section cutthrough Section 3-3 in FIG. 2.

While an embodiment of the invention is described and illustrated hereinhaving a certain number of sensors 320, eight for example, configured asa full Wheatstone bridge (full-bridge), it will be appreciated that thisis for illustration purposes only and that the scope of the invention isnot so limited and encompasses any number of sensors suitable for apurpose disclosed herein, such as two or four, for example, and may beconfigured as a full-bridge or a half-bridge having electrical circuitryknown in the art and therefore not requiring further description orillustration herein. As such, reference hereinafter is simply to the“Bridge 322”, which serves to encompass both a full-bridge and ahalf-bridge, or more particularly at least a partial Wheatstone bridge.

With reference now to FIG. 3 in combination with FIGS. 1, 2 and 2A, thestrain gauge 100 further includes: at least one dielectric layer 200,which is part of the composite of the plurality of layers 104, disposedon top of the substrate 102; a strain sensor layer 300, which is part ofthe composite of the plurality of layers 104, having a thin filmelectrical circuit 302, in the configuration of the Bridge 322, disposedon top of the at least one dielectric layer 200 and having at least twoinput terminals 304, 306 and at least two output terminals 308, 310(best seen with reference to FIGS. 2 and 2A); and, a plurality of layers400, which is part of the composite of the plurality of layers 104,disposed on top of the electrical circuit 302.

In an embodiment, the plurality of layers 400 comprises two layers; adielectric a-layer 400.1 disposed on top of the electrical circuit 302,and an encapsulant b-layer 400.2 disposed on top of the a-layer 400.1.In an embodiment, the a-layer 400.1 comprises an oxide, such as but notlimited to glass for example, and the b-layer 400.2 comprises a curableorganic encapsulant, such as but not limited to silicone for example.

In an embodiment, the at least one dielectric layer 200 comprises twolayers; a c-layer 200.1 disposed on top of the substrate 102, and ad-layer 200.2 disposed on top of the c-layer 200.1. In an embodiment,the c-layer 200.1 comprises a native oxide derived from the substrate102, such as but not limited to chrome oxide for example, and thed-layer 200.2 comprises an oxide, such as but not limited to glass forexample.

From the foregoing, it will be appreciated that the thin film electricalcircuit 302 of the Bridge 322 is disposed in an open-face relationshipwith an environment (i.e., ambient) external of the strain gauge 100,while at the same time being encapsulated by the at least one dielectriclayer 200 and the plurality of layers 400.

The at least one dielectric layer 200 forms a first moisture resistantseal 202 on one side 312, the underside, of the electrical circuit 302,and the plurality of layers 400 forms a second moisture resistant seal402 on a second side 314, the upperside, of the electrical circuit 302opposite the first side 312.

In an embodiment, the at least one dielectric layer 200 is disposed inconforming intimate contact with an upper surface 112 of the substrate102, and the plurality of layers 400 is disposed in conforming intimatecontact with the upper surface (i.e., upperside) 314 of the electricalcircuit 302. In an embodiment, the upper surface 112 of the substrate102 is a polished surface upon which the at least one first dielectriclayer 200 is disposed. As used herein, the phrase “in conformingintimate contact” means that the respective abutting materials have noor substantially no voids therebetween so as to provide a degree ofmoisture resistant seals suitable for a purpose disclosed herein.

For the strain gauge 100 to have the structural integrity to withstandexposure to multiple autoclave surgical-instrument sterilizing cyclesand multiple surgical-instrument cleaning detergent cycles withoutstatistically significant loss in accuracy, the first and secondmoisture resistant seals 202, 402 have sufficient structure such that:in a first instance prior to exposure of the strain gauge 100 to anautoclave cycle, the electrical circuit 302 is productive of a firstoutput voltage on the output terminals 308, 310 in response to a firstpredefined input voltage on the input terminals 304, 306; and, in asecond instance subsequent to exposure of the strain gauge 100 to atleast 25 autoclave cycles, alternatively to at least 50 autoclavecycles, or further alternatively to at least 100 autoclave cycles, theelectrical circuit 302 is productive of a second output voltage on theoutput terminals 308, 310 in response to a second predefined inputvoltage on the input terminals 304, 306, where the second input voltageis equal to the first input voltage within a defined acceptance range,and the second output voltage is equal to the first output voltagewithin a defined acceptance range. In an embodiment, the second outputvoltage being equal to the first output voltage is representative of theplurality of electrically resistive thin film sensors 320 of the Bridge322 being free of strain and absent a corrosion-influenced electricalshort circuit path that would otherwise be effective to cause the Bridge322 to become unbalanced. In an embodiment, the equivalence of thesecond input voltage to the first input voltage, and the equivalence ofthe second output voltage to the first output voltage, may beestablished through experimentation and statistical analysis, where theterm equivalence is understood to mean statistically equivalent, thatis, any observed difference between two measured values is considered tobe an acceptable difference for a purpose disclosed herein.

In an embodiment, each autoclave cycle is configured to expose thestrain gauge 100 to saturated steam at a relative humidity of 100%, at apressure of 2-35 psi (pounds per square inch), at a temperature of100-140 degree-Celsius, for a length of time of 15-20 minutes. Morespecifically, each autoclave cycle is configured to expose the straingauge 100 to saturated steam at a relative humidity of 100%, at apressure of 16-30 psi, at a temperature of 121-138 degree-Celsius, for alength of time of 17-19 minutes. While certain lengths of times aredescribed herein for an autoclave cycle, it will be appreciated thatthis is for illustration purposes only, as a particular sterilizationprocess may require more or less time, where some processes may requirea lesser length of time of 90-180 seconds, for example, or may require alength of time greater than 20 minutes, for example.

In an embodiment, each surgical-instrument cleaning detergent cyclesubjects the strain gauge 100 to a substance having a pH level of equalto or greater than 9 pH. In an embodiment, the pH level of the detergentsubstance is equal to or less than 11 pH. In an embodiment, the firstinstance is prior to exposure of the strain gauge 100 to asurgical-instrument cleaning detergent cycle, and the second instance issubsequent to exposure of the strain gauge 100 to at least 25surgical-instrument cleaning detergent cycles, alternatively to at least50 surgical-instrument cleaning detergent cycles, or furtheralternatively to at least 100 surgical-instrument cleaning detergentcycles.

To withstand the aforementioned autoclave cycles and/orsurgical-instrument cleaning detergent cycles without statisticallysignificant loss in accuracy, not only does the electrical circuit 302of the electrically conductive strain sensor layer 300 have to beadequately sealed to prevent a corrosion-influenced electrical shortcircuit path that would otherwise be effective to cause the Bridge 322to become unbalanced, but also the input and output terminals 304, 306,308, 310 have to be adequately sealed while permitting solderablecontacts to be electrically connected thereto. In an embodiment, aplurality of solderable contacts 324, 326, 328, 330 are disposed onrespective ones of the input and output terminals 304, 306, 308, 310 ontop of the conductive layer 300, each of the at least two inputterminals 304, 306 and the at least two output terminals 308, 310 beingin electrical connection with a respective one of the plurality ofsolderable contacts 324, 326, 328, 330, where the plurality ofsolderable contacts 324, 326, 328, 330 are physically accessible for asolder connection 114 via respective open regions 404 absent the atleast one second dielectric layer 400.

The open regions 404 absent the at least one second dielectric layer 400may be formed in at least two different ways.

In a first way, the plurality of layers 400, and more specifically thea-layer 400.1, is conformally applied to seal the entire surface of theelectrically conductive strain sensor layer 300, including thepreviously applied solderable contacts 324, 326, 328, 330. Vias (alsoherein referred to by reference numeral 404) are then etched through thea-layer 400.1 to form the respective open regions 404, thereby providingfor the plurality of solderable contacts 324, 326, 328, 330 to bephysically accessible via respective vias 404 through the a-layer 400.1of the plurality of layers 400, the respective vias 404 being therespective open regions 404 absent material of the plurality of layers400. The b-layer 400.2 is then conformally applied to further seal theunderlying materials, including the solder connection 114. FIG. 3depicts the entire solderable contact 324 disposed completely on therespective terminal 304 of the electrically conductive strain sensorlayer 300, and the a-layer 400.1 completely sealing around the perimeterof the solderable contact 324. The other solderable contacts 326, 328,330 are formed the same way.

In a second way, and with reference to alternative strain gauge 100′depicted in FIG. 4 where like features are numbered alike, thesolderable contact 324′ extends beyond an outer perimeter of theelectrically conductive strain sensor layer 300, being disposedpartially on the respective terminal 304, and partially on the at leastone dielectric layer 200. Also depicted in FIG. 4 is the solderablecontact 324′ being disposed in overlapping relationship with an outeredge of the a-layer 400.1. In this embodiment, accessibility to thesolderable contact 324′ may be created by masking the radially outerregion of the solderable contact 324′ prior to conformally applying thea-layer 400.1 of the plurality of layers 400, thereby leaving exposedthe open regions 404 upon removal of the mask after the a-layer 400.1 isconformally applied, resulting in the plurality of solderable contacts(contact 324′ for example) being physically accessible via therespective open regions 404 disposed proximate an outer edge of thea-layer 400.1. The b-layer 400.2 is then conformally applied to furtherseal the underlying materials, including the solder connection 114.

A third option for exposing the solderable contacts is to mask thecontact region while depositing the encapsulating a-layer 400.1. In thisembodiment, accessibility to the solderable contact 324′ may be createdby placing the mask directly on the substrate 102′ prior to conformallyapplying the a-layer 400.1 of the plurality of layers 400. Removal ofthe mask would then leave the solderable contacts un-encapsulated thuseliminating the need to etch vias.

While FIG. 3 depicts only one solderable contact 324, it will beappreciated that the other solderable contacts 326, 328, 330 depicted inFIG. 2 are similarly constructed. And while FIG. 4 depicts only onesolderable contact 324′, it will be appreciated that three othersolderable contacts, synonymous to contacts 326, 328, 330, may besimilarly constructed.

With respect to either the embodiment depicted in FIG. 3 or theembodiment depicted in FIG. 4, an embodiment further includes respectiveones of a plurality of electrical wires 118 electrically connected torespective ones of the plurality of solderable contacts, such as thesolderable contacts 324, 324′ or any other such solderable contact asdescribed herein for example. The electrical wires 118 may beelectrically connected to the solderable contacts via solder 114, or byany other means suitable for a purpose disclose herein, such asultrasonic welding for example. In an embodiment, the electrical wires118 are insulated 34 AWG (American Wire Gauge).

Alternative to the use of electrical wires 118, a flex circuit, such asa flexible printed circuit board for example, may be employed. The flexcircuit may be soldered or wire bonded to the solderable contacts.

Referring back to FIG. 2, an embodiment of the strain gauge 100 includesan arrangement where the plurality of electrically resistive thin filmsensors 320 of the Bridge 322 are arranged relative to each other and toan orthogonal set of X-Y axis such that: a first pair of the sensors320.1, 320.2 have mirror image symmetry with respect to the X-axis; asecond pair, different from the first pair, of sensors 320.3, 320.4 havemirror image symmetry with respect to the X-axis; a third pair,different from the first and the second pair, of sensors 320.1, 320.3have mirror image symmetry with respect to the Y-axis; and, a fourthpair, different from the first, the second, and the third pair, ofsensors 320.2, 320.4 have mirror image symmetry with respect to theY-axis. Another embodiment of the strain gauge 100 includes anarrangement where the plurality of electrically resistive thin filmsensors 320 of the Bridge 322 are further arranged relative to eachother and to the orthogonal set of X-Y axis such that: a fifth pair,different from the first, the second, the third, and the fourth pair, ofthe sensors 320.5, 320.6 have mirror image symmetry with respect to theX-axis; a sixth pair, different from the first, the second, the third,the fourth pair, and the fifth pair, of sensors 320.7, 320.8 have mirrorimage symmetry with respect to the X-axis; a seventh pair, differentfrom the first, the second, the third, the fourth pair, the fifth, andthe sixth pair, of sensors 320.5, 320.7 have mirror image symmetry withrespect to the Y-axis; and, an eighth pair, different from the first,the second, the third, the fourth pair, the fifth, the sixth, and theseventh pair, of sensors 320.6, 320.8 have mirror image symmetry withrespect to the Y-axis.

With reference to FIGS. 1 and 2 in combination, an embodiment of thestrain gauge 100 includes an arrangement where the plurality ofelectrically resistive thin film sensors 320 of the Bridge 322 areradially disposed with respect to the Z-axis between the engagementsurface 106 and an outer body 120 of the substrate 102 in the region110. In an embodiment, respective pairs of the sensors 320 arediametrically opposed with respect to each other, as depicted in FIG. 1by sensor pairs: 320.1 and 320.4; 320.2 and 320.3; 320.5 and 320.8; and,320.6 and 320.7, for example.

While an embodiment of the strain gauge 100 is described and illustratedherein having sensors 320 arranged relative to each other in aparticular geometric pattern, it will be appreciated that this is forillustration purposes only and that the scope of the invention is not solimited, as different strain gauge applications may be better servedhaving the sensors 320 arranged relative to each other in a differentgeometric pattern, such as being oriented in an aligned lineararrangement, oriented in an orthogonal arrangement, or oriented in aradial arrangement, for example. As such, any geometric pattern ofsensors 320 suitable for a purpose disclosed herein is considered tofall within the scope of the invention disclosed herein.

Operation of the strain gauge 100 involves the outer body 120 being heldfixed relative to the engagement surface 106 while the force F, or moregenerally the pressure P, is applied to the engagement surface 106,which results in the upperside 108, and particularly the region 110where the plurality of electrically resistive thin film sensors 320 ofthe Bridge 322 are disposed, being subjected to a strain. Flexing of theelectrically conductive strain sensor layer 300 causes the Bridge 322 tobecome unbalanced. Knowing the strain sensitivity of the strain gauge100, one can determine the applied strain, and force F or pressure P,from the change in gauge resistance using known techniques. If one ormore of the plurality of electrically resistive thin film sensors 320experience a change in resistivity due to one or morecorrosion-influenced electrical short circuit paths, which may resultfrom the extreme environment of an autoclave cycle for example, then theapplied strain and force F or pressure P will not be able to beaccurately determined. As such, it is the role of the at least onedielectric layer 200 and the plurality of layers 400 to provide thenecessary bond and moisture seal to protect the electrically conductivestrain sensor layer 300 of the Bridge 322 from corrosion, while at thesame time being flexible enough to permit the electrically conductivestrain sensor layer 300 of the Bridge 322 to flex under the appliedstrain. While not being held to any particular theory, Applicant hasfound that sandwiching the electrically conductive strain sensor layer300 of the Bridge 322 between at least one first bonded oxide layer,with reference to reference numeral 200, and at least one second bondedoxide layer, with reference to reference numeral 400, having structureas disclosed herein, provides the desired protection for the straingauge 100 to survive multiple autoclave cycles and multiplesurgical-instrument cleaning detergent cycles. More particularly,Applicant has found that using an oxide for the at least one dielectriclayer 200, and using a combination of silicone on top of an oxide forthe b-layer 400.2 and the a-layer 400.1, respectively, provides thedesired protection for the strain gauge 100 to survive 25, 50 or even100 autoclave cycles.

As a result and as disclosed herein, some embodiments of the inventionmay include some of the following advantages: a strain gauge capable ofwithstanding the environment of 25, 50 or 100 autoclave cycles withoutstatistically significant loss in accuracy; and, a strain gauge capableof withstanding the environment of 25, 50 or 100 surgical-instrumentcleaning detergent cycles without statistically significant loss inaccuracy.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best oronly mode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope of theappended claims. Also, in the drawings and the description, there havebeen disclosed exemplary embodiments of the invention and, althoughspecific terms may have been employed, they are unless otherwise statedused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention therefore not being so limited.Moreover, the use of the terms first, second, etc. do not denote anyorder or importance, but rather the terms first, second, etc. are usedto distinguish one element from another. Furthermore, the use of theterms a, an, etc. do not denote a limitation of quantity, but ratherdenote the presence of at least one of the referenced item.

What is claimed is:
 1. An open-face strain gauge, comprising: asubstrate; at least one dielectric layer disposed on top of thesubstrate; a thin film electrical circuit disposed on top of the atleast one dielectric layer and having at least two input terminals andat least two output terminals; a plurality of layers disposed on top ofthe electrical circuit; the at least one dielectric layer forming afirst moisture resistant seal on one side of the electrical circuit, theplurality of layers forming a second moisture resistant seal on a secondside of the electrical circuit opposite the first side, the first andsecond moisture resistant seals having structure such that: in a firstinstance prior to exposure of the strain gauge to an autoclave cycle,the electrical circuit is productive of a first output voltage on theoutput terminals in response to a first input voltage on the inputterminals; and in a second instance subsequent to exposure of the straingauge to at least 25 autoclave cycles, the electrical circuit isproductive of a second output voltage on the output terminals inresponse to a second input voltage on the input terminals, the secondinput voltage being equal to the first input voltage, and the secondoutput voltage being equal to the first output voltage.
 2. The straingauge of claim 1, wherein the second instance is subsequent to exposureof the strain gauge to at least 50 autoclave cycles.
 3. The strain gaugeof claim 2, wherein the second instance is subsequent to exposure of thestrain gauge to at least 100 autoclave cycles.
 4. The strain gauge ofclaim 1, wherein each autoclave cycle is configured to expose the straingauge to saturated steam at a relative humidity of 100%, at a pressureof 2-35 psi, at a temperature of 100-140 degree-Celsius, for a length oftime of 15-20 minutes.
 5. The strain gauge of claim 4, wherein theautoclave cycle is configured to expose the strain gauge to saturatedsteam at a pressure of 16-30 psi, at a temperature of 121-138degree-Celsius, for a length of time of 17-19 minutes.
 6. The straingauge of claim 1, wherein: the substrate comprises a corrosion resistantmetal; the at least one dielectric layer is disposed in conformingintimate contact with an upper surface of the substrate; the electricalcircuit comprises a conductive layer having a plurality of electricallyresistive thin film sensors electrically arranged to form at least apartial Wheatstone bridge having the at least two input terminals andthe at least two output terminals; and the plurality of layers isdisposed in conforming intimate contact with an upper surface of theelectrical circuit.
 7. The strain gauge of claim 6, further comprising:a plurality of solderable contacts disposed on top of the conductivelayer, each of the at least two input terminals and the at least twooutput terminals being in electrical connection with a respective one ofthe plurality of solderable contacts, the plurality of solderablecontacts being physically accessible via respective open regions absentmaterial of the plurality of layers.
 8. The strain gauge of claim 1,wherein: the plurality of layers comprises two layers; an a-layerdisposed on top of the electrical circuit and a b-layer disposed on topof the a-layer, the a-layer comprising an oxide, and the b-layercomprising a curable organic encapsulant.
 9. The strain gauge of claim8, wherein the a-layer oxide comprises glass, and the b-layer comprisesa curable organic encapsulant comprising silicone.
 10. The strain gaugeof claim 6, wherein the second output voltage being equal to the firstoutput voltage is representative of the plurality of electricallyresistive thin film sensors of the at least a partial Wheatstone bridgebeing free of strain and absent a corrosion influenced electrical shortcircuit path effective to cause the at least a partial Wheatstone bridgeto become unbalanced.
 11. The strain gauge of claim 8, wherein theplurality of solderable contacts are physically accessible viarespective vias through the a-layer, the respective vias being therespective open regions absent material of the a-layer.
 12. The straingauge of claim 8, wherein the plurality of solderable contacts arephysically accessible via the respective open regions disposed proximatean outer edge of the a-layer.
 13. The strain gauge of claim 7, furthercomprising: respective ones of a plurality of electrical wireselectrically connected to respective ones of the plurality of solderablecontacts.
 14. The strain gauge of claim 13, wherein the electrical wiresare electrically connected to the solderable contacts via solder. 15.The strain gauge of claim 1, wherein: the at least one dielectric layercomprises two layers; a c-layer disposed on top of the substrate and ad-layer disposed on top of the c-layer, the c-layer comprising a nativeoxide, and the d-layer comprising an oxide.
 16. The strain gauge ofclaim 15, wherein the c-layer native oxide comprises chrome oxide, andthe d-layer oxide comprises glass.
 17. The strain gauge of claim 1,wherein the substrate comprises a polished upper surface upon which theat least one first dielectric layer is disposed.
 18. The strain gauge ofclaim 6, wherein the conductive layer comprises a resistive alloy, andthe electrically resistive thin film sensors of the at least a partialWheatstone bridge comprise electrical paths defined by etched or lasertrimmed portions of the resistive alloy.
 19. The strain gauge of claim1, wherein the thin film electrical circuit is disposed in an open-facerelationship with an environment external of the strain gauge.
 20. Thestrain gauge of claim 1, further wherein: the first instance is prior toexposure of the strain gauge to a surgical-instrument cleaningdetergent; and the second instance is subsequent to exposure of thestrain gauge to at least 25 cleaning cycles comprising thesurgical-instrument cleaning detergent.
 21. The strain gauge of claim 6,wherein: the plurality of electrically resistive thin film sensors ofthe at least a partial Wheatstone bridge are arranged relative to eachother and to an orthogonal set of X-Y axes such that: a first pair ofthe sensors have mirror image symmetry with respect to the X-axis; asecond pair, different from the first pair, of sensors have mirror imagesymmetry with respect to the X-axis; a third pair, different from thefirst and the second pair, of sensors have mirror image symmetry withrespect to the Y-axis; and, a fourth pair, different from the first, thesecond, and the third pair, of sensors have mirror image symmetry withrespect to the Y-axis.
 22. The strain gauge of claim 21, wherein theplurality of electrically resistive thin film sensors of the at least apartial Wheatstone bridge are further arranged relative to each otherand to the orthogonal set of X-Y axes such that: a fifth pair, differentfrom the first, the second, the third, and the fourth pair, of thesensors have mirror image symmetry with respect to the X-axis; a sixthpair, different from the first, the second, the third, the fourth pair,and the fifth pair, of sensors have mirror image symmetry with respectto the X-axis; a seventh pair, different from the first, the second, thethird, the fourth pair, the fifth, and the sixth pair, of sensors havemirror image symmetry with respect to the Y-axis; and, an eighth pair,different from the first, the second, the third, the fourth pair, thefifth, the sixth, and the seventh pair, of sensors have mirror imagesymmetry with respect to the Y-axis.
 23. The strain gauge of claim 6,wherein the substrate further comprises: an overall circular footprinthaving a central Z-axis; and an engagement surface at an undersideregion of the substrate; wherein the plurality of electrically resistivethin film sensors of the at least a partial Wheatstone bridge areradially disposed with respect to the Z-axis between the engagementsurface and an outer perimeter of the substrate.